1. Field of the Invention
The present invention broadly relates to a magnetic head device mounted to a rotary drum of a video tape recorder (VTR) capable of variable speed reproduction, and, more particularly, to a magnetic head device whose head tip is displaced and driven in a predetermined direction by using an electromechanical converting element (bimorph plate).
2. Description of the Related Art
A so-called DT (dynamic tracking) head that tracks recording tracks during reproduction at variable speed has been used as a magnetic head device which is mounted to a rotary drum of a conventional video tape recorder (VTR).
FIG. 4 is a view showing a conventional magnetic head device using the above-described DT head.
In the magnetic head device, a parallel leaf spring member 8 and a head tip 9 are supported by two supporting members, that is by a dummy base 10 and a bimorph plate 6. A flexible base 12 is affixed on top of the dummy base 10 by a screw 4.
As shown in FIG. 4A, the bimorph plate 6 is formed from a trapezoidal sheet, and, as shown in FIG. 4B, it is sandwiched and held by a fixed portion 11 and a fixed plate 7, with an end side being a free end that is joined to a bottom leaf spring 8b of the parallel leaf spring member 8. The dummy base 10 is formed from a rectangular sheet. Its base end portion is integrally formed with the fixed portion 11 fixed at a location corresponding to the location of the fixed plate 7. An end of the dummy base 10 is a free end that is joined to an upper leaf spring 8a of the parallel leaf spring member 8.
The bimorph plate 6 is adhered to a pair of piezoelectric ceramic plates through a reinforcer using an epoxy type adhesive. The piezoelectric ceramic plates (made of lead zirconate, lead titanate, or the like) are formed from sintered ceramic powder. Electrode layers, such as those formed by a silver-baked paint electrode, are formed on each of the surfaces thereof. The magnetic head device is provided with a drive circuit (not shown) in order to apply a voltage V to such a bimorph plate 6 through the electrode layers described above.
The dummy base 10 is made of carbon-type material (carbon fiber type material) and has a resilience that allows it to deform sufficiently in accordance with the movement of the bimorph plate 6.
In such a construction, applying a voltage to the bimorph plate 6 from the electrode layers causes one of the piezoelectric plates adhered to the bimorph plate 6 to expand (or contract) and the other piezoelectric plate to contract (or expand).
This causes the bimorph plate 6 to bend as a whole. Here, the dummy base 10, which is connected to the bimorph plate 6 through the parallel plate spring member 8, bends in accordance with the bending of the bimorph plate 6. Thus, while being supported by the two supporting members (the bimorph plate 6 and the dummy base 10), the head tip 9 can be displaced in the direction of arrow a or in a direction opposite thereto by merely driving only one bimorph plate 6.
The amplitude of the displacement in the forward or reverse direction is approximately 600 .mu.m. Displacing the head tip 9 in this way allows reproducing operations, from reverse reproduction to a reproduction performed three times faster, without any noise.
When the head tip 9 is displaced in the forward or reverse direction so as to move with an amplitude, the parallel plate spring member 8 acts to improve the contact between the head tip 9 and the tape. Like the dummy base 10, the parallel leaf spring member 8 is made of carbon-type material (carbon fiber).
In FIG. 4A, reference numeral 13 denotes wirings, and reference numerals 14 and 15 denote lands to which the wirings 13 are connected.
However, in such a conventional magnetic head device, the dummy base 10 is adhered to the upper leaf spring 8a of the parallel leaf spring member 8, as shown in FIG. 5C, after only part of the lower leaf spring 8b of the parallel leaf spring member 8 and part of the bimorph plate 6 are placed upon each other, as shown in FIG. 5A, and adhered together from the front and back sides of the lower leaf spring 8b and the bimorph plate 6, using epoxy resin p, as shown in FIG. 5B. Thus, the conventional magnetic head device has the following problems:
(a) It is necessary to apply a large amount of epoxy resin p in order to adhere the lower leaf spring 8b of the parallel leaf spring member 8 and the bimorph plate 6 together with a predetermined adhesive strength, thereby increasing the number of manhours.
(b) Since the lower leaf spring 8b of the parallel leaf spring member 8 and the bimorph plate 6 are adhered together merely by placing them on top of each other, the state of adhesion between the spring 8b and the bimorph plate 6 becomes unstable, with variations in the adhesive strength, which causes variations in the displacement amplitude of the head tip 9.
(c) In addition, since the lower leaf spring 8b of the parallel leaf spring member 8 and the bimorph plate 6 are adhered together merely by placing them on top of each other, variations occur in the posture of the parallel leaf spring member 8 adhered to the bimorph plate 6. This causes the head tip 9 disposed at the parallel leaf spring member 8 to tilt, since it cannot assume a most suitable posture, so that the angle of a gap formed at the head tip 9 no longer matches the angle of the tape recording lines. This results in poorer reproduction output characteristics (or electromagnetic conversion characteristics), since the angle of the gap is set less accurately with respect to the required azimuth.